A variant form of the type C atrial natriuretic peptide receptor generated by alternative RNA splicing

Natriuretic peptides: their structures, receptors, physiologic functions and therapeutic applications

Natriuretic peptides are a family of three structurally related hormone/ paracrine factors. Atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) are secreted from the cardiac atria and ventricles, respectively. ANP signals in an endocrine and paracrine manner to decrease blood pressure and cardiac hypertrophy. BNP acts locally to reduce ventricular fibrosis. C-type natriuretic peptide (CNP) primarily stimulates long bone growth but likely serves unappreciated functions as well. ANP and BNP activate the transmembrane guanylyl cyclase, natriuretic peptide receptor-A (NPR-A). CNP activates a related cyclase, natriuretic peptide receptor-B (NPR-B). Both receptors catalyze the synthesis of cGMP, which mediates most known effects of natriuretic peptides. A third natriuretic peptide receptor, natriuretic peptide receptor-C (NPR-C), clears natriuretic peptides from the circulation through receptor-mediated internalization and degradation. However, a signaling function for the receptor has been suggested as well. Targeted disruptions of the genes encoding all natriuretic peptides and their receptors have been generated in mice, which display unique physiologies. A few mutations in these proteins have been reported in humans. Synthetic analogs of ANP (anaritide and carperitide) and BNP (nesiritide) have been investigated as potential therapies for the treatment of decompensated heart failure and other diseases. Anaritide and nesiritide are approved for use in acute decompensated heart failure, but recent studies have cast doubt on their safety and effectiveness. New clinical trials are examining the effect of nesiritide and novel peptides, like CD-NP, on these critical parameters. In this review, the history, structure, function, and clinical applications of natriuretic peptides and their receptors are discussed.

Agonistic induction of a covalent dimer in a mutant of natriuretic peptide receptor-A documents a juxtamembrane interaction that accompanies receptor activation

The natriuretic peptide receptor-A (NPR-A) is composed of an extracellular domain with a ligand binding site, a transmembrane-spanning domain, a kinase homology domain, and a guanylyl cyclase domain. In response to agonists (atrial natriuretic peptide (ANP) and brain natriuretic peptide), the kinase homology domain-mediated guanylate cyclase repression is removed, which allows the production of cyclic GMP. Previous work from our laboratory strongly indicated that agonists are exerting their effects through the induction of a juxtamembrane dimeric contact. However, a direct demonstration of this mechanism remains to be provided. As a tool, we are now using the properties of a new mutation, D435C. It introduces a cysteine at a position in NPR-A corresponding to a supplementary cysteine found in NPR-C6, another receptor of this family (a disulfide-linked dimer). Although this D435C mutation only leads to trace levels of NPR-A disulfide-linked dimer at basal state, covalent dimerization can be induced by a treatment with rat ANP or with other agonists. The NPR-A(D435C) mutant has not been subjected to significant structural alterations, since it shares with the wild type receptor a similar dose-response pattern of cellular guanylyl cyclase activation. However, a persistent activation accompanies NPR-A(D435C) dimer formation after the removal of the inducer agonist. On the other hand, a construction where the intracellular domain of NPR-A(D435C) has been truncated (DeltaKC(D435C)) displays a spontaneous and complete covalent dimerization. In addition, the elimination of the intracellular domain in wild type DeltaKC and DeltaKC(D435C) is associated with an increase of agonist binding affinity, this effect being more pronounced with the weak agonist pBNP. Also, a D435C secreted extracellular domain remains unlinked even after incubation with rat ANP. In summary, these results demonstrate, in a dynamic fashion, the agonistic induction of a dimeric contact in the juxtamembrane domain of NPR-A. In addition, this process seems to require membrane attachment of the receptor. Finally, the intracellular domain represses this contact at the basal state, showing its potent influence on the outer juxtamembrane domain.

Guanylyl cyclases as a family of putative odorant receptors

Mammals can discriminate among a large number (> 10,000) of unique odorants. The most highly supported explanation for this ability is that olfactory neurons express a large number of seven transmembrane receptors that are not spatially organized at the level of the olfactory epithelium, but whose axonal projections form a distinct pattern within the olfactory bulb. The odor-induced signaling pathway in olfactory neurons includes a Gs-like protein (G(olf)) that activates a specific adenylyl cyclase (type III) isoform, resulting in elevations of cyclic AMP and subsequent activation of a cyclic nucleotide-gated channel. The channel also can be regulated by cyclic GMP. Recently, an olfactory neuron-specific guanylyl cyclase was discovered in rodents, and subsequently a large family of sensory neuronal guanylyl cyclases was identified in nematodes. These guanylyl cyclases are concentrated in the plasma membrane of the dendritic cilia and contain extracellular domains that retain many of the primary sequence characteristics of guanylyl cyclases known to be receptors for various peptides. Thus, the guanylyl cyclases appear to represent a second family of odorant/pheromone receptors.

A disulfide-bridged mutant of natriuretic peptide receptor-A displays constitutive activity. Role of receptor dimerization in signal transduction

Natriuretic peptide receptor-A (NPR-A), a particulate guanylyl cyclase receptor, is composed of an extracellular domain (ECD) with a ligand binding site, a transmembrane spanning, a kinase homology domain (KHD), and a guanylyl cyclase domain. Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP), the natural agonists, bind and activate the receptor leading to cyclic GMP production. This receptor has been reported to be spontaneously dimeric or oligomeric. In response to agonists, the KHD-mediated guanylate cyclase repression is removed, and it is assumed that ATP binds to the KHD. Since NPR-A displays a pair of juxtamembrane cysteines separated by 8 residues, we hypothesized that the removal of one of those cysteines would leave the other unpaired and reactive, thus susceptible to form an interchain disulfide bridge and to favor the dimeric interactions. Here we show that NPR-AC423S mutant, expressed mainly as a covalent dimer, increases the affinity of pBNP for this receptor by enhancing a high affinity binding component. Dimerization primarily depends on ECD since a secreted NPR-A C423S soluble ectodomain (ECDC423S) also documents a covalent dimer. ANP binding to the unmutated ECD yields up to 80-fold affinity loss as compared with the membrane receptor. However, the ECD C423S mutation restores a high binding affinity. Furthermore, C423S mutation leads to cellular constitutive activation (20-40-fold) of basal catalytic production of cyclic GMP by the full-length mutant. In vitro particulate guanylyl cyclase assays demonstrate that NPR-AC423S displays an increased sensitivity to ATP treatment alone and that the effect of ANP + ATP joint treatment is cumulative instead of synergistic. Finally, the cellular and particulate guanylyl cyclase assays indicate that the receptor is desensitized to agonist stimulation. We conclude the following: 1) dimers are functional units of NPR-A guanylyl cyclase activation; and 2) agonists are inducing dimeric contact of the juxtamembranous region leading to the removal of the KHD-mediated guanylyl cyclase repression, hence allowing catalytic activation.

Molecular cloning of two rat GRK6 splice variants

Desensitization of G protein-coupled receptors is frequently triggered by G protein-coupled receptor kinases (GRKs) that preferentially phosphorylate agonist-occupied receptors. In this study, two GRK6 splice variants were cloned from the rat kidney. One isoform (GRK6a) encodes a 576-amino acid protein that is virtually identical (98% identity) to human GRK6. The second isoform is similar except for a 2-base pair insert that constitutes part of an intron interrupting the 3'-end coding region. This new isoform (GRK6b, 589 amino acids) has therefore a specific COOH-terminal region. A reverse transcription-polymerase chain reaction assay designed to discriminate GRK6 splice variants demonstrated that GRK6b mRNA is widely distributed and expressed at much higher levels than GRK6a mRNA in most peripheral tissues. In contrast, GRK6a predominates in brain. Functional studies, performed with cytosol extracts from transfected Chinese hamster ovary cells, indicated that GRK6a and GRK6b both phosphorylate light-activated rhodopsin as well as a synthetic peptide. The identification of GRK6b extends the family of GRKs. Further studies will be required to establish the tissue and subcellular distribution of this protein and to delineate its physiological role.

Guanylyl cyclase expression in specific sensory neurons: a new family of chemosensory receptors

A guanylyl cyclase (GC-D) was recently shown to be expressed in a subclass of neurons within the neuroepithelim of the rat, but given that only a single cyclase was discovered, whether it represents an odorant/pheromone receptor as has been suggested for the large family of seven-transmembrane receptors remains unclear. Through cloning and expression of cDNA we now demonstrate that at least 29 genomic or cDNA sequences found in Caenorhabditis elegans represent guanylyl cyclases. Many of the membrane forms retain cysteine residues conserved within the extracellular, ligand-binding domain of known cyclase receptors. Of eight orphan cyclase receptor::GFP (green fluroescence protein) fusion constructs for which signals were obtained, all were expressed in specific sensory neurons. Furthermore, a cyclase/GFP fusion protein (GCY-10/GFP) was principally expressed in the sensory cilium, suggesting these cyclases function as primary chemosensory receptors. For the first time, we also found that chemosensory neurons (ASE), known to be bilaterally symmetric, demonstrate absolute right or left sidedness with respect to the expression of three different cyclases. Thus, the guanylyl cyclases represent an unexpectedly large and new family of sensory neuron receptors that may complement the 7-transmembrane family of odorant/pheromone receptors.

Alternative splicing of natriuretic peptide A and B receptor transcripts in the rat brain

1. In the present study we searched for variants of alternative splicing of guanylyl cyclase A and B mRNA in rats in vivo. 2. Guanylyl cyclase A2 and guanylyl cyclase B2 isoforms of guanylyl cyclase produced by alternative splicing leading to the deletion of exon 9 of both transcripts were quantified in several rat organs. 3. Only one alternative splicing was found in the regulatory domain, encoded by exons 8-15. 4. Quantification of the guanylyl cyclase B2 isoform in different rat organs and in cultured aortic smooth muscle cells showed that this alternative splicing was tissue-specific and occurred predominantly in the central nervous system where the alternatively spliced variant represented more than 50% of the guanylyl cyclase B mRNA. 5. The same alternative splicing existed for guanylyl cyclase A mRNA but at very low levels in the organs studied. 6. Alternative splicing of guanylyl cyclase B exon 9 in the brain may play an important role in signal transduction, since the expressed protein possesses a constitutionally active guanylyl cyclase acting independently of C-type natriuretic peptide regulation.

Cloning and properties of a novel natriuretic peptide receptor, NPR-D

A novel natriuretic peptide receptor, which we have termed natriuretic peptide receptor D (NPR-D), has been cloned and characterized. cDNAs related to the natriuretic peptide receptor (NPR) were amplified by PCR from a template of poly(A)-rich RNA isolated from the eel gill. Sequencing of the PCR products revealed the presence of a new clone that showed about 70% sequence identity to the eel type-C receptor, NPR-C. The PCR fragment was used to determine the tissue distribution of the new NPR-D message by an RNase protection assay, which gave the strongest signal in brain samples, and then used to screen a brain library to obtain a full-length cDNA clone. The cDNA clone predicted a protein of 500 amino acids containing a signal sequence and a hydrophobic transmembrane segment. The predicted sequence also contained the NPR motif which is essential for the binding of natriuretic peptides. The protein NPR-D was expressed in COS cells and shown to have high affinities for eel and rat natriuretic peptides. The newly cloned NPR-D has a short cytoplasmic tail; in this respect, NPR-C and NPR-D are very similar and form a subfamily of the NPR family. Affinity labeling indicated that NPR-D exists as a disulfide-linked tetramer. This is a marked contrast to the homodimeric structure of NPR-C. HS-142-1, a non-peptide natriuretic peptide receptor antagonist of microbial origin previously shown to be selective for the guanylate-cyclase-coupled receptors NPR-A and NPR-B, competitively inhibited the binding of 125I-labeled eel natriuretic peptide to eel NPR-D, whereas it did not affect the binding activity of eel NPR-C, suggesting that HS-142-1 is an antagonist that recognizes the tetrameric structures of NPR since the guanylate-cyclase-coupled receptors have also been demonstrated to exist as tetramers.

Cloning, properties, site-directed mutagenesis analysis of the subunit structure, tissue distribution and regulation of expression of the type-C eel natriuretic peptide receptor

Eel natriuretic peptide receptor C (NPR-C) was cloned, characterized and found to have a unique interchain disulfide linkage when compared to that of mammalian NPR-C. The NPR-C cDNA was obtained from an eel gill cDNA library; the open reading frame codes for a polypeptide of 502 amino acids exhibiting the known features of NPR-C, including a weak ligand specificity and a disulfide-linked homodimeric structure. The deduced amino acid sequence shares approximately 60% similarity with the mammalian NPR-C but it lacks the Gly-rich prosequence present in the mammalian counterparts. Site-directed mutagenesis revealed that eel and mammalian NPR-C are quite different in their interchain disulfide-bonding pattern; eel uses the second Cys residue and mammals the fifth Cys residue for the covalent dimerization. The ligand-binding activity of the extracellular domain is not independent of the short cytoplasmic tail. RNase protection analysis revealed that the eel receptor is highly expressed in the gill and heart and, to a much lesser extent, in other tissues including the brain and intestine. The NPR-C mRNA levels were found to be down-regulated in most tissues when eels were transferred from fresh water to seawater; however, in the anterior intestine, the levels were up-regulated, suggesting that NPR-C plays a role in the adaptation to salinity changes in the euryhaline eel.

Molecular analysis of vasopressin receptors in the rat nephron. Evidence for alternative splicing of the V2 receptor

Expression and regulation of vasopressin V2 and V1a receptors were studied at the mRNA level in the rat kidney. Two V2 mRNA variants were identified and shown to arise from a single gene by alternative splicing using one donor and two different acceptor sites. The long (V2L) form encodes the adenylyl cyclase-coupled receptor. The short (V2S) form lacks the nucleotide sequence encoding the putative seventh transmembrane domain and undergoes a frame shift in its 3'end coding region; it is inactive on the cyclase pathway in transfected cells. Measurement of mRNAs, carried out by quantitative reverse transcription-polymerase chain reaction (RT-PCR) on microdissected nephrons, demonstrated that neither V2L, V2S nor V1A mRNAs are expressed in glomeruli and proximal tubules (< 100 mRNA copies/glomerulus or mm of tubular length), whereas they are present in the ascending limb of Henle's loop and in the collecting tubule. The V2L mRNA, which is always predominant in these structures, is expressed throughout the collecting tubule at 10 times higher levels (30,000 copies/mm) than in the thin and thick ascending limbs. The ratio of the V2S over V2L mRNA is constant (15%) in all nephron segments; hence high V2S levels are only observed in the collecting tubule. The V1A mRNA is slightly expressed in the thin ascending limb, absent in the thick ascending limb and reaches its maximum in the cortical collecting duct (4,000 copies/mm), before gradually decreasing to undetectable levels in the terminal collecting duct. Finally, in vivo administration of a vasopressin V2 agonist decreased by 50% V2L and V2S mRNAs, but did not alter the V1A mRNA level. We conclude that this study provides the quantitation, on a molar basis, of vasopressin receptor mRNAs in kidney tubules and demonstrates the occurrence of two V2 mRNA spliced variants which are similarly down-regulated.

Cloning and expression of eel natriuretic-peptide receptor B and comparison with its mammalian counterparts

A comparative study of the natriuretic-peptide receptor NPR-B was performed by cloning and expressing, in COS-1 cells, the NPR-B receptor subtype from the eel gill which exhibited a strong C-type-natriuretic-peptide (CNP)-induced guanylate cyclase activity. Like other mammalian NPR-B receptors, the eel NPR-B receptor consisted of a ligand-binding extracellular domain, a hydrophobic transmembrane domain, a kinase-like domain and a guanylate cyclase domain. Sequence comparison among the eel and mammalian receptors revealed a relatively low similarity (approximately 44%) in the extracellular domain compared to a very high similarity (approximately 84%) in the cytoplasmic regulatory and catalytic domains. This low similarity allowed identification of the amino acid residues or candidate regions important for the ligand-binding activity. RNase protection analysis of the eel NPR-B mRNA demonstrated that the message was predominantly expressed in the liver and atrium as well as in the gill with moderate-to-small amounts in the brain, ventricle, esophageal sphincter, stomach, posterior intestine and kidney. The high NPR-B mRNA levels in the liver, atrium and gill were found to decrease markedly when eels were transferred from fresh water to seawater and kept there for 2 weeks. Since similar changes are known to occur in the ligand CNP levels when eels are facing osmotic challenges, the CNP/NPR-B system appears to play an important role in their successful adaptation to salinity changes.